Image: J-Pole Antenna constructed by Nico ZS4N (Click image for larger view.)
The J-pole antenna, more properly known as the J antenna, is a vertical omnidirectional transmitting antenna. It was invented by Hans Beggerow in 1909 for use in Zeppelin airships. Trailed behind the airship, it consisted of a single one half wavelength long wire radiator, in series with a quarter-wave parallel transmission line tuning stub that matches the antenna impedance to the feed line. By 1936 this antenna began to be used for land-based transmitters with the radiating element and the matching section mounted vertically, giving it the shape of the letter "J", and by 1943 it was named the J antenna. The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feed line by a shorted quarter-wave parallel transmission line stub. For a transmitting antenna to operate efficiently, absorbing all the power provided by its feed line, the antenna must be impedance matched to the line; it must have a resistance equal to the feed line's characteristic impedance. A half-wave antenna fed at one end has a current node at its feed point, giving it a very high input impedance of around 1 000–4 000 ohms. This is much higher than the characteristic impedance of transmission lines, so it requires an impedance matching circuit between the antenna and the feed line. A shorted quarter-wave stub, a transmission line one quarter of the wavelength long with its conductors shorted together at one end, has a similar high impedance node at its open end, making a good match to the antenna. The input impedance seen at a point along the stub varies continuously, decreasing monotonically from this high value to zero at the shorted end. So any value of input impedance can be obtained by connecting the feed line to the proper point along the stub. One arm of the stub is extended a half wavelength to make the antenna. By attaching the antenna's feed line to the proper point along the transmission line, the stub will transform this impedance down to match the lower feed line impedance, allowing the antenna to be fed power efficiently. During construction the proper attachment point for the feed-line is found by sliding the connection of the feed line back and forth along the stub while monitoring the SWR until an impedance match (minimum SWR) is obtained.[1][6] Being a half-wave antenna, it provides a small gain of just under 1 dB over a quarter-wave ground-plane antenna. Primarily a dipole, the J-pole antenna exhibits a mostly omnidirectional pattern in the horizontal (H) plane with an average free-space gain near 2.2 dBi (0.1 dBd).
In July 2024 Nico ZS4N decided to build a J-Pole Antenna for the 10m and 11m band. Nico indicated that he received better reports with the J-Pole Antenna than the 3 Element Delta Loop Antenna for 26 - 30 Mhz he built in May 2024 available HERE. More information and images available beneath.
Image: J-Pole Antenna constructed by Nico ZS4N (Click images for larger view.)
More information about the Slim and J Pole Calculator available HERE
More information available HERE
